Cavity antenna mounted on a missile

ABSTRACT

An antenna for use on a missile wherein a cylindrical conductor is concentrically positioned about a metallic surface portion of the missile so as to define a cavity of one-quarter wavelength between one end of the cylindrical conductor, which is connected with the missile surface, and a plurality of connecting positions of energy transfer means. The energy transfer means is connected about the periphery of the cylindrical conductor at intervals substantially equal to a single wavelength of a signal so that the cylindrical conductor and the missile surface adjacent an opposite end of the cylindrical conductor form the elements of an asymmetric dipole of high impedance.

United States Patent 1 1 Munson 111 3,7 10,338 [451 Jan. 9, 1973 15 41 cAv1TY ANTENNA MOUNTED ON A MISSILE [75] Inventor: Robert E. Munson, Boulder, Colo.

[73] Assignee: Ball Brothers Research Corporation,

Boulder, C010.

[22] Filed: Dec. 30, 1970 211 Appl. No.: 102,983

Related US. Application Data [63] Continuation of Ser. No. 787,912, Dec. 30, 1968,

abandoned.

[52] US. Cl. ..343/708, 343/769, 343/807 [51] Int. Cl. ..H0lq 1/28 [58] Field of Search ..343/705, 708, 769, 807

[56] References Cited UNITED STATES PATENTS 3,365,721 1/1968 QBittner ..343/70s 2,990,546 6/1961 Haas ..343/767 3,128,467 4 19 4 Lanctot 343/708 3,226,720 12/1965 Brunner et a1 ..343/708 Primary Examiner-Eli Lieberman Attorney-Campbell, Harris & ORourke 57 ABSTRACT An antenna for use on a missile wherein a cylindrical conductor is concentrically positioned about a metallic surface portion of the missile so as to define a cavity of one-quarter wavelength between one end of the cylindrical conductor, which is connected with the missile surface, and a plurality of connecting positions of energy transfer means. The energy transfer means is connected about the periphery of the cylindrical conductor at intervals substantially equal to a single wavelength of a signal so that the cylindrical conductor and the missile surface adjacent an opposite end of the cylindrical conductor form the elements of an asymmetric dipole of high impedance.

7 Claims, Drawing Figures SIGNAL SOURCE PATENTEDJAN 9m 3.710.333

SHEET 1 BF 2 SIGNAL SOURCE INVENTOR.

ROBERT E. MUNSON OWN-4&2,

A TTOIP/VEYS PATENTEDJAH 9 ms 3. 7 l O 3 38 sum 2 [IF 2 ROBERT E. MUNSON BY WM M/QLrOWM ATTORNEYS CAVITY ANTENNA MOUNTED ON A MISSILE This application is a continuation of copending U. S. application Ser. No. 787,912, filed Dec. 30, 1968, now abandoned, entitled Cavity Antenna by Robert E. Munson.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to antennas and more particularly to an improved cavity antenna particularly useful for integration with a missile.

2. Description of the Prior Art Presently, rockets are being utilized for carrying instrument payloads for short term measurement of various high altitude environmental data. The various sensor instrumentation often operates in conjunction with a transmitting unit for instantaneous and continuous data transmittal to one or more ground tracking stations. However, constant monitoring has been difficult due to signal nulls encountered as the vehicle assumes different roll and aspect orientations.

Although automatic gain control amplifiers have to some extent alleviated the problem of antenna deficiencies, the need to improve the antennas so as to develop radiation patterns without signal lobes of varying strength has nevertheless been recognized as the fundamental problem. More particularly, an antenna should be characterized by an isotropic antenna radiation coverage, that is, a pattern of constant relative power for any orientation of the antenna. As such, the pattern coverage is then relatively constant regardless of the roll or aspect orientation of the rocket thereby facilitating data monitoring at a tracking station. The avoidance of signal nulls as a characteristic of the antenna precludes one important factor that often previously has caused temporary loss of signal information;

and in an unrecovered rocket the permanent loss.

It has most always been recognized that an antenna onsupersonic vehicles such as rockets must satisfy harsh aerodynamic design requirements. For example, the antenna should not' substantially disrupt the mechanical structure of the rocket; otherwise, its ruggedness is decreased. Further, the antenna design mustnot substantially increase air drag.

Heretofore, attempts toward solving or obviating the aforementioned problems have not been rapidly forthcoming due, at least in part, to other concentrated effort of antenna designs for use as guided missile type weaponsQIn such limited use, it usually is not required that the missile antenna have a constant power level relative to isotropic radiation but often only that a high sensitivity threshold exist forwardly of the missile and in the direction of the target.

Although some previous attempts have also been made to design antennas having patterns more nearly isotropic for uses in rockets carrying environmental data sensors and associated instruments for telemetry purposes and the like, the antennas have been less than satisfactory in either failing to satisfy the strict and aforementioned aerodynamic design requirements or in exhibiting intolerable signal variations in the aspect patterns (the signal pattern measured about the missile in a plane containing the missile), and/or in the roll patterns (the signal pattern measured about the missile in a plane perpendicular to the missile axis). The aspect and roll radiation patterns in antennas of even relatively recent design have often fluctuated as much as 30 db from isotropic radiation.

SUMMARY OF THE INVENTION The present invention overcomes these and other disadvantages and provides an antenna having an improved signal radiation pattern.

The antenna, particularly useful in airborne telemetry vehicles which assume many orientations relative to any given tracking station, has a low profile to avoid substantial increase of air drag and is integrated in the missile so as to maintain the mechanical strength of the missile body.

A cylindrical conductor is connected as a short circuit at one end to the skin of the missile and is concentrically positioned therewith so as to define a cavity. A plurality of coaxial transmission lines are connected to the cylindrical conductor and missile at a distance from the shorted end of substantially one-quarter wavelength of the radio frequency signal to be propagated. In effect, a fat" asymmetrical dipole (a dipole which in cross section is large in physical size in terms of the wavelength of the signal) is formed by the cylindrical conductor and the skin of the missile, which dipole develops substantially uniform aspect and roll radiation patterns.

It is accordingly an object of the invention to provide a novel antenna having an improved signal radiation pattern.

It is another object of the present invention to provide an improved antenna integrated in a missile.

Another object of the invention is to provide an antenna utilizing the missile skin as a multi-wavelength asymmetric dipole.

A further object of the invention is to provide an antenna integrated in a missile to form an asymmetric dipole without appreciably disrupting the continuity of the missile surface and support structure or appreciably increasing weight and/or air drag on the missile.

Another object of the invention is to provide an antenna having a reduced radiation pattern variation about the roll axis.

Another object of the invention is to provide an antenna having a reduced radiation pattern variation about the aspect axis.

A further object of the invention is to provide an antenna integrated in a rocket having a circumference substantially greater than the wavelength of signals to be transmitted and/or received by the antenna wherein an asymmetric dipole is formed by one portion of the missile body, and a cylindrical conductor and another portion of the missile body, the conductor being connected at one end to the missile body and concentrically extending therewith.

These and other objects and advantages will be apparent to those skilled in the art from the following description of a preferred embodiment of the invention as shown in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a perspective view of a missile having the antenna of this invention thereon;

FIG. 2 is a broken-away partial side view of the missile and antenna shown in FIG. 1., but greatly enlarged with respect thereto;

FIG. 3 is a diagrammatic illustration of electromagnetic energy radiation from the antenna integrated in the missile as shown in FIGS. 1 and 2; and

FIG. 4 is a graphic representation showing an experimental antenna gain radiation pattern utilizing the outer surface of a missile as a fat asymmetric dipole.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the embodiment of the invention as illustrated in FIG. 1, an antenna is shown integrated in a missile 12 having a metallic outer cylindrical body, or skin, 14 and a nose portion 15.

As best shown in FIG. 2, antenna 10 includes a cylindrical conductor 16, preferably of brass, which is concentrically positioned with respect to the missile and spaced outwardly therefrom by a layer of dielectric material 18 characterized by low electrical loss, for example, polytetrafluoro ethylene (commercially available Teflon).

The cylindrical conductor 16 is curved inwardly toward the forward portion of the missile to be received in an annular notch 20 of a conductor 22. Conductor 22 is a ring concentrically mounted about the missile and is secured thereto by a plurality of spaced screws, such as screw 23. As shown in FIG. 2, conductor 22 is substantially triangular in cross-section with notch 20 being cut at the edge of one leg so that a continuous smooth outer surface is presented along the entire missile protuberance formed by conductors 16 and 22, to thus minimize the effects of air drag in the widening transition area.

The cylindrical conductor 16, conductor 22, and the forward portion of the missile 10 form one element of a fat asymmetric dipole (as explained hereinafter) with the aft portion of the vehicle body 14 as the other element. Conductor 16 also forms, along with a portion of the missile body 14, an annular cavity 24 which contains dielectric material 18. As hereinafter described, the axial length of cavity 24, as corrected for the impedance producing effect of the dielectric layer 18 therein, is approximately one-fourth of the carrier signal wavelength at the anticipated frequency of operation. The cavity may be excited by signal energy produced at a source 25 within the missile l2 and which may be of conventional type; for example, a unit having an appropriate power source and a radio frequency oscillator modulated in accordance with data signals from external environmental sensor devices.

The signal source 25 is connected by a coaxial line 26 to an N-way power divider 28 wherein N represents the numerical number of output legs each of which is connected to a different one of a plurality of coaxial lines, such as lines 30 and 32, as shown in FIG. 2. As is known in the art, such dividers may be employed over wide frequency bands to separate input R. F. signals into two or more equal phase and amplitude output signals and to convey the signals to the individual output legs with very little power loss. A wide range of dividers of this type are commercially available such as from the Microlab/FXR Co., 10 Microlab Road, Livingston, New Jersey. The type of divider depends upon the particular power requirements, frequency of operation, and the like. At a popular telemetry transmission frequency of 2.2 GHZ on a rocket approximately 15 inches in diameter, for example, it would be preferable to excite the cavity 24 at 14 or more feed positions as more fully hereinafter described; therefore, the signal from source 25 would be separated by divider 28 into a corresponding number of equal phase and amplitude signal components.

The equal phase and amplitude signals are transferred from the output legs of the divider 28 by respective coaxial lines each having inner conductors such as conductors 34 and 36 and outer conductors, such as conductors 35 and 37, Coaxial line 30 and connections therewith are representative of the other coaxial lines and respective connections. Near the termination of line 30, a ring 40 is fixed to the line. An internally threaded female connector 42 having a shoulder 43 is received over ring 40 and is connected to a mounting jack 44 by means of external mating threads on the mounting jack with shoulder 43 engaging ring 40 to hold the line in position. As shown broken away in FIG. 2, inner conductor 34 terminates in a hollow sleeve portion 45 which extends into mounting jack 44 when line 30 is held in position by connector 42 on jack 44.

The mounting jack is also of conventional type and includes an integral rectangular flange 46 having holes to receive screws 48 for attachment of the jack to the missile body 14. Jack 44 has a center conductor 50 one end of which is received into and is tightly engaged with sleeve 45 when the coaxial line is held positioned by connector 42 in jack 44. Body portion 52 of the jack is formed by conductive material and has a recess 53 therein to receive the outer conductor 35 of line 30 to thereby form an electrical connection therewith when the line is positioned in the jack by connector 42. Dielectric material 54, which is preferably of the same material as the dielectric material in the coaxial line 30 fills the space between the center conductor 50 and jack body portion'52.

As shown in FIG. 2, jack 44 also has an annular neck 55 which extends from flange 46 through a hole provided in the missile body. Center conductor 50 extends through neck 55 and dielectric material 18 to cylindrical conductor 16. Preferably, the cylindrical conductor is provided with a hole 56 slightly larger than conductor 50 so as to permit conductor 50 to be received and fastened therein, as for example, by soldering from a position outside the antenna 10 in order that the dielectric layer 18 is not disturbed.

In operation, it may be readily appreciated that the radio frequency signal energy is transferred to the cavity 24 effectively by a plurality of continuous coaxial lines from the power divider 28 with the cavity excited in the TEM mode. The cylindrical conductor 16 and the portion of the missile body 14 forming cavity 24 has an impedance characteristic of effectively a onequarter wavelength coaxial transmission line which is short-circuited at the end opposite the feed positions by conductor 22.

As is well known, the impedance produced by a cavity ofa given length with air as the dielectric in the cavity is different from the effective impedance presented by the cavity when a dielectric material other than air is contained in the cavity. The relation of effective wavelength as a function of actual wavelength is:

wherein A, is the corrected wavelength, A is the actual wavelength in air at the frequency of operation and e is the dielectric constant, relative to a vacuum, of the material in the cavity.

Thus, for operation at a carrier frequency of 2.2 GHz and utilizing a dielectric such as polymerized tetrafluoro ethylene, as in the aforenoted example, A is approximately 5.4 inches and a is approximately 2.5 inches. Solving the equation according to the above yields:

A, 5.4 inches/ 2.5

= 3.4 inches Therefore, the effective one-quarter wavelength cavity for the exemplary parameters stated is slightly less than 1 inch in length when polymerized tetrafluoro ethylene is the dielectric material in the cavity. It may readily be appreciated that at the given carrier frequency and for the rocket diameter of the example (i.e., 2.2 GHz and 15 inches, respectively), the circumference of the rocket is much greater than A namely, approximately 14 times greater.

It has been realized that the most favorable radiation patterns are developed when the cavity 24 is excited in the TEM mode with signals of uniform phase and amplitude provided at feed positions which are separated about the periphery of the cylindrical conductor 16 at distances corresponding to substantially one wavelength as corrected for the dielectric material 18 in the cavity. Therefore, in the example, since the rocket 12 is approximately 14 effective wavelengths in circumference, a l4-way power divider 28, or combination of power dividers to obtain 14 equal phase and amplitude outputs, as already suggested, would be utilized and I4 inner conductors, such as inner conductor 50 of respective jacks, such as jack 44, would be connected to the cylindrical conductor 16 at positions equally spaced one from the other about the periphery of the body portion 14.

Since the corrected wavelength (A is much less than the circumference of the rocket 12, the cylindrical conductor 16 and the rocket body 14 form a fat asymmetric dipole. If the rocket had a small circumference relative to the corrected wavelength, deep nulls would exist in the aspect radiation pattern; that is, a plurality of lobes would appear in the pattern with deep nulls therebetween. Since this undesirable condition is equivalent to exciting the cavity 24 with signal energy at a lower frequency, it has been found that the antenna of the present invention operates better as frequency is increased, instead of worse as is the case in utilizing discrete radiators.

It should be apparent that the short circuit at the forward end of the cavity presents a negligible impedance which effectively transforms back as an open circuit at the feed positions; namely, at the positions of the conductors such as inner conductor 50 which conductors are each connected to the cylindrical conductor 16 at peripheral position substantially equal to A /4 rearwardly from conductor 22. The apparent open circuit is effectively in parallel with the radiation impedance of the fat asymmetrical dipole. Since the antenna is large in diameter in terms of wavelength, the impedance across the asymmetrical dipole is real and divides between the feed positions.

FIG. 3 shows a broken-away, reduced portion of the antenna 10 of FIG. 1 integrated in rocket 12 wherein the dotted lines between the cylindrical conductor 16 and the aft portion of the missile body 14 illustrate the signal energy emanating from the antenna 10 and more particularly the approximate direction of the electric field existing between the elements of the dipole at a particular instant in time. The direction of the field is reversed (not shown) at every half wavelength from the rearward end of the cavity 24 in both the fore and aft directions since the polarity of cylindrical conductor 16 relative to the aft portion of body 14 is constantly changing in accordance with the frequency of the oscillatory signal.

Referring to FIG. 4, the aspect radiation pattern developed by an antenna utilizing the outer surface of a missile as a fat asymmetric dipole is shown wherein the axis of the missile was positioned substantially in the plane containing the pattern. The pattern, representing the antenna ga'in relative to linear isotropic radiation, is substantially representative within 1 db of the infinite number of aspect radiation patterns which may be utilized to define a figure of revolution about the missile axis. More particularly, the radiation pattern of FIG. 4 may be substantially represented by the pattern obtained in any plane defining a cross section through the figure of revolution produced by all aspect patterns and containing the axis of the rocket.

It may readily be appreciated that deep nulls exist only directly forwardly (0) and rearwardly from the tip and tail of the rocket, respectively. As already noted, tip and tail pattern nulls in a telemetry rocket are usually of little concern. In the pattern shown, the area subtended due to the tip and tail nulls is less than one-hundredth of one percent of a total spherical surface enclosure about the aforementioned figure of revolution.

As can also be seen from FIG. 4, the remainder of the signal pattern displays an average strength variation between signal peaks and nulls in the aspect plane of less than 5 db. Thus, the improved antenna is highly favorable for the receipt or transmission of electromagnetic signal energy to or from the missile without any appreciable average loss in signal due to vehicle orientation.

Although only one embodiment of the invention has been shown and described, various modifications as may appear to those skilled in the art are intended to be within the contemplation of the invention as defined in scope by the claims.

What is claimed is:

1. In a propelled vehicle having a cylindrical body, such as a missile, an antenna assembly comprising: an electrically conductive cylindrical element positioned concentrically about and spaced from a portion of said cylindrical body, said cylindrical element having a diameter greater than the diameter of said portion of said cylindrical body with which it is concentric and an axial length substantially equal to one-quarter wavelength at an anticipated operating frequency of said assembly; electrically conductive means for connecting an entire circumferential edge of said cylindrical element to said portion so as to define a coaxial cavity, one end of which is completely opened and one end which is completely closed; and electrical signal feed means connected to said element and said portion at a point substantially adjacent the open end of said cavity.

2. An assembly according to claim 1 wherein said cylindrical element and at least a part of said cylindrical body form an asymmetrical dipole of large diameter relative to one wavelength at the anticipated operating frequency of said assembly.

3. In a propelled vehicle having a cylindrical body, such as a missile, an antenna assembly comprising: an electrically conductive cylindrical element positioned concentrically about and spaced from a portion of said cylindrical body, said cylindrical element having a diameter greater than the diameter of said portion of said cylindrical body with which it is concentric; electrically conductive means for connecting an entire circumferential edge of said cylindrical element to said portion so as to define a coaxial cavity, one end of which is completely opened and one end which is completely closed; and electrical signal feed means connected to said element and said portion, said feed means being connected to said cylindrical element and said portion of said cylindrical body at a plurality of substantially equally circumferential spaced points, the circumferential distance between any two adjacent points being approximately one wavelength at an anticipated operating frequency of said assembly.

4. In a propelled vehicle having a cylindrical body, such as a missile, an antenna assembly comprising: an electrically conductive cylindrical element positioned concentrically about and spaced from a portion of said cylindrical body, said element having a diameter greater than the diameter of said portion of said cylindrical body with which it is concentric and having an axial length substantially equal to one-quarter wavelength at the anticipated operating frequency of said assembly; electrically conductive meansfor connecting an entire circumferential edge of said cylindrical element to said portion so as to define a coaxial cavity, one end of which is completely opened and one end of which is completely closed; and electrical signal feed means including a plurality of leads connected to said element and said portion of said cylindrical body at points substantially adjacent the open end of said cavity, said points being equally circumferentially spaced at intervals approximately equal to one wavelength at said operating frequency.

5. [n a missile having a circumference greater than a plurality of wavelengths, in a given dielectric, of a signal, an integrated antenna comprising: a cylindrical conductor concentrically positioned about a body portion of the missile and connected at one end with the missile so as to form a continuous circumferential cavity about the portion of the missile said cavity having one end completely opened and the other end completely closed; and a plurality of coaxial transmission lines each having an inner conductor and an outer conductor, the inner conductors each being connected with the cylindrical conductor at peripheral intervals equal at least to substantially one wavelength of the signal, in the given dielectric, and the outer conductors being connected with the missile, both the inner and outer conductors of respective coaxial lines being spaced from the one end at substantially one-quarter wavelength of the signal, in the given dielectric, so that said cylindrical conductor and missile form a high impedance cavity, the impedance being in parallel with radiation impedance existing substantially between said cylindrical conductor and missile body.

6. An antenna assembly comprising: an inner electrically conductive cylindrical element having an axial length at least substantially equal to one-quarter wavelength at the anticipated operating frequency of said assembly; an outer electrically conductive cylindrical element positioned concentrically about and spaced from at least a portion of said inner cylindrical element, said outer cylindrical element having a diameter greater than the diameter of said portion of said inner cylindrical element with which it is concentric and an axial length substantially equal to one-quarter wavelength at said anticipated operating frequency; electrically conductive means for connecting an entire circumferential edge of said outer cylindrical element to said portion of said inner cylindrical element so as to define a coaxial cavity, one end of which is completely opened and one end of which is completely closed; and electrical signal feed means connected to said cylindrical elements at a point substantially adjacent the open end of said cavity.

7. An antenna assembly comprising: an inner electri cally conductive cylindrical element; an outer electrically conductive cylindrical element positioned concentrically about and spaced from at least a portion of said inner cylindrical element, said outer cylindrical element having a diameter greater than the diameter of said portion of said inner cylindrical element with which it is concentric and an axial length substantially equal, at most, to the axial length of said inner cylindrical conductor; electrically conductive means for connecting an entire circumferential edge of said outer cylindrical element to said portion of said inner cylindrical element so as to define a coaxial cavity, one end of which is completely opened and one end of which is completely closed; and electrical signal feed means connected to said cylindrical elements, said feed means being connected to said cylindrical elements at a plurality of substantially equally circumferential spaced points, the circumferential distance between any two adjacent points being, at most, approximately one wavelength at an anticipated operating frequency of said assembly. 

1. In a propelled vehicle having a cylindrical body, such as a missile, an antenna assembly comprising: an electrically conductive cylindrical element positioned concentrically about and spaced from a portion of said cylindrical body, said cylindrical element having a diameter greater than the diameter of said portion of said cylindrical body with which it is concentric and an axial length substantially equal to one-quarter wavelength at an anticipated operating frequency of said assembly; electrically conductive means for connecting an entire circumferential edge of said cylindrical element to said portion so as to define a coaxial cavity, one end of which is completely opened and one end which is completely closed; and electrical signal feed means connected to said element and said portion at a point substantially adjacent the open end of said cavity.
 2. An assembly according to claim 1 wherein said cylindrical element and at least a part of said cylindrical body form an asymmetrical dipole of large diameter relative to one wavelength at the anticipated operating frequency of said assembly.
 3. In a propelled vehicle having a cylindrical body, such as a missile, an antenna assembly comprising: an electrically conductive cylindrical element positioned concentrically about and spaced from a portion of said cylindrical body, said cylindrical element having a diameter greater than the diameter of said portion of said cylindrical body with which it is concentric; electrically conductive means for connecting an entire circumferential edge of said cylindrical element to said portion so as to define a coaxial cavity, one end of which is completely opened and one end which is completely closed; and electrical signal feed means connected to said element and said portion, said feed means being connected to said cylindrical element and said portion of said cylindrical body at a plurality of substantially equally circumferential spaced points, the circumferential distance between any two adjacent points being approximately one wavelength at an anticipated operating frequency of said assembly.
 4. In a propelled vehicle having a cylindrical body, such as a missile, an antenna assembly comprising: an electrically conductive cylindrical element positioned concentrically about and spaced from a portion of said cylindrical body, said element having a diameter greater than the diameter of said portion of said cylindrical body with which it is concentric and having an axial length substantially equal to one-quarter wavelength at the anticipated operating frequency of said assembly; electrically conductive means for connecting an entire circumferential edge of said cylindrical element to said portion so as to define a coaxial cavity, one end of which is completely opened and one end of which is completely closed; and electrical signal feed means including a plurality of leads connected to said element and said portion of said cylindrical body at points substantially adjacent the open end of said cavity, said points being equally circumferentially spaced at intervals approximately equal to one wavelength at said operating frequency.
 5. In a missile having a circumference greater than a plurality of wavelengths, in a given dielectric, of a signal, an integrated antenna comprising: a cylindrical conductor concentrically positioned about a body portion of the missile and connected at one end with the missile so as to form a continuous circumferential cavity about the portion of the missile said cavity having one end completely opened and the other end completely closed; and a plurality of coaxial transmission lines each having an inner conductor and an outer cOnductor, the inner conductors each being connected with the cylindrical conductor at peripheral intervals equal at least to substantially one wavelength of the signal, in the given dielectric, and the outer conductors being connected with the missile, both the inner and outer conductors of respective coaxial lines being spaced from the one end at substantially one-quarter wavelength of the signal, in the given dielectric, so that said cylindrical conductor and missile form a high impedance cavity, the impedance being in parallel with radiation impedance existing substantially between said cylindrical conductor and missile body.
 6. An antenna assembly comprising: an inner electrically conductive cylindrical element having an axial length at least substantially equal to one-quarter wavelength at the anticipated operating frequency of said assembly; an outer electrically conductive cylindrical element positioned concentrically about and spaced from at least a portion of said inner cylindrical element, said outer cylindrical element having a diameter greater than the diameter of said portion of said inner cylindrical element with which it is concentric and an axial length substantially equal to one-quarter wavelength at said anticipated operating frequency; electrically conductive means for connecting an entire circumferential edge of said outer cylindrical element to said portion of said inner cylindrical element so as to define a coaxial cavity, one end of which is completely opened and one end of which is completely closed; and electrical signal feed means connected to said cylindrical elements at a point substantially adjacent the open end of said cavity.
 7. An antenna assembly comprising: an inner electrically conductive cylindrical element; an outer electrically conductive cylindrical element positioned concentrically about and spaced from at least a portion of said inner cylindrical element, said outer cylindrical element having a diameter greater than the diameter of said portion of said inner cylindrical element with which it is concentric and an axial length substantially equal, at most, to the axial length of said inner cylindrical conductor; electrically conductive means for connecting an entire circumferential edge of said outer cylindrical element to said portion of said inner cylindrical element so as to define a coaxial cavity, one end of which is completely opened and one end of which is completely closed; and electrical signal feed means connected to said cylindrical elements, said feed means being connected to said cylindrical elements at a plurality of substantially equally circumferential spaced points, the circumferential distance between any two adjacent points being, at most, approximately one wavelength at an anticipated operating frequency of said assembly. 